It is not just about what the Scots can produce for themselves. It is about exports. That which has made by far the biggest difference to the Scottish economy is the licensing of oilfields and the taxation on the sale of the oil produced. There is finite oil and nearly all the easy to get at
oil has been brought up. It will be difficult / expensive to get at the remainder and in any event oil is going to be phased out as a source of energy.

This makes Scotland unlikely to be a long term independent successful economy. The time for Scotland to become independent was 50 years ago, for it then to reap the rewards of oil for 50 years and during that time to use such massive amounts of money to create an economy not based upon oil.

This is what the Arab states are doing and what the UK failed to do, but the UK economy will always be a better prospect than an independent Scottish economy.